In some types of power transistor devices, a variable known as the specific on-resistance RSP is a meaningful figure of merit. In essence, RSP tells a designer how much area is needed for a device to realize a certain resistance. In mathematical terms, Rsp=Rdson*area. For example, if a device has an on-resistance of 100 mΩ and an area of 1 mm2, it would have an RSP of 100 mΩ*mm2. Thus, if the designer had a new circuit that needed an on-resistance of 200 mΩ, the designer could cut the area of the device in half (i.e., use a device with an area of 0.5 mm2). Ideally, designers would like RSP to be as small as possible.